1. Field of the Invention
The present invention relates generally to estimating optical insertion loss, and more particularly, to methods and apparatus for estimating insertion loss at a mechanical joining point between optical fibers.
2. Technical Background
Insertion loss at fiber joining points within an optical network should be determined to be within acceptable limits in order to verify proper physical contact between adjoining optical fibers and to maintain the system loss budget. Currently, one specified way of determining insertion loss is with the use of a hand-held optical loss test set, which measures the insertion loss through a length of optical fiber that may include one or more joining points between adjoining optical fibers. For example, a typical insertion loss measurement involves evaluating a length of optical fiber having a mated connector pair at each end of the optical fiber link being tested. However, the condition of the test jumpers and the test equipment referencing procedure (if implemented improperly) can negatively influence the accuracy of the measurement. As a result, a connector is sometimes needlessly replaced due to measurement error although the actual insertion loss over the optical fiber link is within acceptable limits. A means to readily, accurately and cost effectively estimate the insertion loss at a fiber joining point during the installation and termination process would greatly increase terminated fiber yield rates and reduce connector scrap rates.
Another conventional method of determining optical power loss when two adjoining optical fibers are mechanically joined is accomplished using an “Optical Time Domain Reflectometer” (OTDR). An OTDR system provides an approximation of the insertion loss (i.e., backscattered optical power as a function of time) by introducing pulses of laser light into one end of an arbitrarily long optical fiber, and then detecting the amount of light that returns. The OTDR continuously measures reflected power as a function of time to display the distance to an optical discontinuity (e.g., mated connector pair; mechanical splice; fiber break; etc.). If an abrupt index of refraction change occurs at the location of the discontinuity, a relatively large amount of optical power will saturate a photodetector for a finite period of time. In order to approximate this type of optical power loss as a function of time, high-speed sampling is required. The recovery time of the OTDR as the sampling and abrupt optical power reception occurs influences how well a particular discontinuity can be uniquely resolved and measured within the level of skill of the operator. For example, an OTDR can be used to determine how well an optical fiber has been terminated, but the approximated optical power loss depends upon the type of measurement method used (e.g., two-point; LSA; etc.) and typically also includes the optical power loss through a length of optical fiber on either side of the fiber joining point in addition to the decrease in optical power due to the mechanical coupling of the adjoining optical fibers. As such, the optical power loss approximation at a fiber joining point obtained using an OTDR is not recognized by industry standards as a direct measurement to determine the condition of the fiber joining point (e.g., termination).
Another method of subjectively estimating insertion loss and determining when adjoining optical fibers have been brought into proper physical contact utilizes a “visual fault locator” (VFL) system. In a typical VFL system, an optical power generator, such as a visible light laser, is used to launch light energy into one of the optical fibers and thereby cause the mechanical joining point to glow. A particular apparatus and method referred to as the “Continuity Test System” (CTS) has been developed by Corning Cable Systems of Hickory, N.C. and is described in greater detail in U.S. Pat. No. 6,816,661. In practice, the primary shortcoming of the CTS method is that the variability in the level of glow, both before and after the termination, creates difficulty in determining the amount of change in the level of glow that is acceptable and indicates a proper termination. Contributing factors include variations in ambient light as well as the operator's subjective interpretation of the change in the level of glow before the optical fibers are joined (reference) glow and after the optical fibers have been joined (terminated) glow. Even after the optical fibers have been successfully joined and an acceptable termination has been made, the splice point may continue to glow slightly, which is referred to in the art as a “nuisance glow.” Oftentimes, an operator will attempt multiple connector terminations or fiber insertions in the same connector/splice in an effort to completely eliminate the nuisance glow. These repeated termination and insertion efforts can result in connector damage and/or optical performance that is less than that which would have been achieved had the operator accepted the first termination, even if the glow was not completely diminished and the nuisance glow persisted.
In view of the shortcomings of the current methods for evaluating insertion loss, improved methods and apparatus are needed for estimating the optical insertion loss at a mechanical joining point between optical fibers. Such methods and apparatus require that optical power be collected and measured at the mechanical joining point of interest under the same conditions during an initial reference measurement and a subsequent insertion loss measurement. Accordingly, the precision of the estimate of optical insertion loss provided by these methods and apparatus, as compared to the optical insertion loss the fiber joining point actually contributes to the total insertion loss, should be limited only by the optical power detection, collection and measurement capabilities of the associated test equipment.